8. ab = cd
.. 3x+4 = 4x-1
.. 5 = x
.. ab = 3*5 +4
.. ab = 19
11. Adjacent vertex angles are supplementary and opposite angles are equal. If the angles are vertex angles numbered in order around the figure,
.. ∠2 = 12°
.. ∠3 = 168°
13. If the quadrilateral is a parallelogram so that point E is the midpoint of both diagonals, it will be a rectangle when the diagonals are equal length.
.. ae = be
.. x +22 = 3x -18
.. 40 = 2x
.. x = 20 . . . . . will make segments AE and BE the same length
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In general, diagrams are helpful for geometry problems.